"Cannot join game because you are not friends with anybody in the game."
or other errors telling you that you that you cant join your friends game because hes not your friend..
Here is the FIX:
--both you and your friend go to xbox.com and make sure you are INDEED friends with each other
--on the website xbox.com, you BOTH send each other a Xbox360 friend request, this is a a drop down under friend request
--refresh page, accept 360 request.
--Hard reset both your xbox ones.
--you will now be able to join your friends server.
No idea what is wrong, but this is the fix. enjoy.